Engineering Geology

Chapter 5
this was indicative of metastable soils. However, Northmore et al. (1996) suggested that a
lower critical value of collapse index, that is 0.22, was more appropriate for some loess type
soils in Essex, England. The double oedometer test also can be used to assess the degree
of collapsibility. The test involves loading an undisturbed specimen at natural moisture content
up to a given load. At this point, the specimen is flooded and the resulting collapse strain,
if any, is recorded. Then, the specimen is subjected to further loading. The total consolidation
upon flooding can be described in terms of the coefficient of collapsibility, C col , given as:
C
= Dh h
col
/
De
=
1+ e
(5.4)
in which Dh is the change in height of the specimen after flooding, h is the height of the specimen
before flooding, De is the change in void ratio of the specimen upon flooding and e is
the void ratio of the specimen prior to flooding. Table 5.11 provides an indication of the potential
severity of collapse. This table indicates that those soils that undergo more than 1%
collapse can be regarded as metastable. However, in China a figure of 1.5% is taken (Lin
and Wang, 1988), and, in the United States, values exceeding 2% are regarded as indicative
of soils susceptible to collapse (Lutenegger and Hallberg, 1988).
Clay Deposits
Clay deposits are composed principally of fine quartz and clay minerals. The three major clay
minerals are kaolinite, illite and montmorillonite. Both kaolinite and illite have non-expansive
lattices, whereas that of montmorillonite is expansive. In other words, montmorillonite is characterized
by its ability to swell and by its notable cation exchange properties.
The microstructure of clay soils is governed largely by the clay minerals present and the forces
acting between them. Because of the complex electrochemistry of clay minerals, the spatial
arrangement of newly sedimented particles is influenced by the composition of the water in which
deposition takes place. Single clay mineral platelets may associate in an edge-to-edge (EE),
edge-to-face (EF), face-to-face (FF) or random type of arrangement, depending on the
Table 5.11. Collapse percentage as an indication of potential problems
Collapse (%)
Severity of problem
0–1 No problem
1–5 Moderate trouble
5–10 Trouble
10–20 Severe trouble
Above 20
Very severe trouble
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